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In this forum we often hear about all sorts of issues related to SD cards formatted as “phone storage” (a.k.a. internal storage). If your SD card is formatted in this way, I strongly recommend that you re-format the card so that it serves as “portable storage” (a.k.a. external storage). Ideally, you should save the data from the card before doing that as formatting will inevitably delete everything on the card. This might be a bit of a hassle, but based on how often we have people here having issues with “phone storage” SD cards, I would strongly assume this is the root cause of your problem with the app, too.
What the two ways to format the SD card entail: